Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., entitled "Method and Apparatus for Modulating a Light Beam". This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are designed to have a thickness equal to .lambda./4 where .lambda. is the wavelength of the incident light source. They are supported a distance of .lambda./4 above, and parallel to, the substrate. When the deformable elements are actuated (for example a sufficient switching voltage is applied), the deformable elements are pulled down and the incident light is diffracted. Optical systems can intercept the diffracted light. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. Furthermore, since gratings are inherently dispersive, this modulator can be used for color displays.
U.S. Pat. No. 5,677,783, issued Oct. 14, 1997 to Bloom et al., entitled "Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate" discloses a method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate. Referring to FIG. 1, a perspective cut-away view of a prior art light modulator 10 is shown. An insulating protective layer 24 of silicon nitride topped with a buffer layer of silicon dioxide 26 is deposited on a silicon substrate 22. This is followed by the deposition of a sacrificial silicon dioxide layer 16. A silicon nitride layer 30 is next deposited in which is defined the deformable elements 12. Both the thickness of the sacrificial silicon dioxide layer 16 and the silicon nitride layer 30 are critical in determining the amplitude modulation and thus the efficiency of the grating device. In order to achieve freestanding beams the sacrificial silicon dioxide layer 16 is etched away in the active area. The remaining sacrificial silicon dioxide layer 16 not removed acts as a supporting frame 14 for the deformable elements 12. The last fabrication step provides an aluminum film (not shown) in order to enhance the reflectance of the beams and provide an electrode for application of a voltage between the deformable elements 12 and the substrate 22.
There are many problems with this prior art device. The thickness of both the sacrificial oxide layer 16 and silicon nitride layer 30 have to each be .lambda./4. Because these thicknesses determine the grating amplitude of the modulator, their dimensions are critical. Variations in either of these thicknesses will result in unwanted diffraction of light in the off state, as well as lower diffraction efficiency in the on state, thus lower contrast ratios. There is no freedom to adjust the thickness of the deformable element 12 for optimization of its mechanical properties.
There is no defined etch stop in the device structure during removal of the sacrificial oxide layer 16. This requires a carefully controlled time-dependent etch to ensure that the remaining sacrificial oxide layer 16 is able act as the supporting frame 14. The profile left by the wet etch openings between the beams leaves an uneven wall below the deformable elements 12 where they contact the supporting frame 14. Such effects will cause variations in the electromechanical properties of the devices.
The etching process to remove the sacrificial oxide layer is also a wet process. During this wet processing step it has been seen that stiction tends to occur in that the deformable elements tend to adhere and remain adhered to the substrate. Special drying techniques can be used to overcome this problem but complicate the process. Removal of the sacrificial layer using a dry process is preferred.
U.S. Pat. No. 5,661,592, issued Aug. 26 ,1997 to Bornstein et al., entitled "Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve" discloses a method for making a deformable grating apparatus which attempts to address the problems associated with this prior art device. An insulating layer is deposited on the substrate. A phosphosilicate glass (PSG) sacrificial layer is next deposited. The PSG sacrificial layer is selectively patterned removing the PSG sacrificial layer except in regions where the deformable grating elements are to be formed. The PSG is reflowed at high temperature to lower the angle of its sidewall. Silicon nitride is then deposited conformably over the PSG and patterned into deformable elements. The PSG sacrificial layer is then removed by wet etching. By selectively patterning the PSG sacrificial layer the region under the beams is more uniform relying now on the uniformity of the reflow of the PSG sacrificial layer. However the removal of the PSG sacrificial layer is still a wet process with the corresponding disadvantages as described above.
The conformal deposition of the silicon nitride over the step height formed by the patterned PSG sacrificial layer region also has topography determined by the step height. In patterning the deformable elements this topography will limit the minimum spacing between the deformable elements. Increased spacing between elements will cause increased light scattering decreasing the efficiency of the grating. The use of a PSG sacrificial layer also requires a high temperature reflow step that would complicate its integration with CMOS circuitry on the same substrate.